I am working on a sketch... but I have a problem... I never understood human anatomy and I'm having problems with it now, too. You can see what I ahve so far... but I don't know how I should draw the body now, would be nice if any of you could help me out.

Woodenmen.. you mean Stiffy Jittz? Ehhe
Find a photo of human in the pose that you wanna draw. Then sketch out very rough and pale outline of the human you're gonna draw. Then refine from there. Redraw parts you don't like,.

I have a picture for the pose the only problem is that the part with the arm is very dark and I can't see how it is looking like. Anyways this will be my first sketch of a body, only drew technical things or heads yet no complete bodies.

But it all depends on the level of realism you are aiming at. The more realistic you go - the more accurate and REAL it should be - therefore - more research and more practice.

I can only tell you this - so far, without any connection to the body -
The GUN - doesn't fit with the arm...
If you look at it for a second you will notice this guy is like, holding it tight IN FRONT of his arm, and not in the same plane with it...
It's like the gun is goes around the arm as a snake...

Try to fix that, and maybe make the gun longer so it sits nicely on the arm...

I'm in desperate need for better realistic skills, so in fact i'm going TOMORROW to purchase a GOOD PILE of books
And some pencil sets...

Anyway - you're on the right direction.
Hopefully - both of us will improve soon!

Woodmen blow goats, go out and do some figure drawing from live models and then buy some books to go along with the classess. Some recomendations are the entire series of George Brigeman's books they're cheap less than six bucks each on Amazon, Drawing lessons from the great masters by Robert Beverly Hale, And Anatomy Lessons by the Great masters by Terence Coyle a student of Hale. and draw your own arms and hands.